1. Field of the Invention
This invention relates to bilateral transmission line repeater circuits and, more particularly, to automatic gain control of the amplifiers in such repeaters.
2. Description of the Prior Art
In transmission systems extending over any considerable distance it is necessary to provide gain along the transmission path in the form of repeaters. Such repeaters utilize automatic gain control circuits to control the signal level of the signals launched on the transmission line segments. Such gain control is necessary to provide standard signal levels at the remote receivers and to prevent overload of the repeater amplifiers during operation.
In subscriber loop carrier systems it is known to control the gain of signals inward bound from remote unattended terminals to the central office terminal by the level of the signal transmitted in the outward bound direction. This arrangement, called "cross control," insures that the signal level of the inward bound signal is normally at a desirable design level at the central office receiver. Since a common transmission line is used for signals in both directions, the level of the outward bound signal at the repeater is an excellent measure of the gain required for the inward bound signal.
Although cross-control gain as described above is normally highly desirable and works well for long-term normal conditions, there are cases when the cable temperature changes are extreme and/or the repeaters are unintentionally placed at wrong intervals along the line, and the cross-control feature by itself is not adequate to guarantee that the inward bound amplifier does not overload.